kHz Particle Image Velocimetry Research Articles

Near-field evolution and scaling of shear layer instabilities in a reacting jet in crossflow

This study analyses the stability characteristics of the shear layer vortices (SLV) in a reacting jet in crossflow, analysing effects of flame position, momentum flux ratio ( $J$ ) and density ratio ( $S$ ). It utilizes 40 kHz particle image velocimetry to characterize the dominant SLV frequencies, streamwise evolution and convective/global stability characteristics for three different canonical configurations, one non-reacting and two reacting (‘R1’ and ‘R2’). In the non-reacting case, both convective and global instability is observed, depending upon $S$ and $J$ . Qualitatively similar $S$ dependencies occur for the R1 reacting case where the radial flame position lies outside the jet shear layer, albeit with slower SLV growth rates. When the flame lies inside the jet shear layer, the R2 reacting case, a qualitatively different behaviour is observed, as vorticity concentration in the shear layers is suppressed almost completely. Finally, we show that frequency and stability characteristics of the non-reacting and R1 cases can be scaled in a unified manner using a counter-current shear layer model. This model relates these SLV behaviours to a vorticity layer thickness, a velocity scale and an effective density ratio (noting that there are three distinct densities associated with the jet, the crossflow and the burned gases). These parameters were extracted from the data and used to collapse the frequency scaling, and to explain the transition to self-excited oscillatory behaviour.

Experimental investigations on central vortex core in swirl spray flames using high-speed laser diagnostics

Centrally staged swirl combustion can effectively reduce NOx emission. However, the complex combustion field is susceptible to producing large-scale coherent structures, such as precessing vortex core and central vortex core (CVC). This study mainly investigates the effect of CVC on the flow and flame in a centrally staged swirl spray combustor at elevated temperature and pressure using 10 kHz high-speed CH* chemiluminescence (CL), 20 kHz particle image velocimetry, and CH2O planar laser-induced fluorescence (PLIF). For the pilot flame, both CH* CL and CH2O PLIF flame are fork-shaped with three long parts, and the middle parts of flame dynamics indicate CVC structure. For the stratified flame, the CVC structure exists in an extended strip area of strong vorticity near the centerline of the combustor. The analysis of proper orthogonal decomposition modes shows that the motion of CVC is mainly swing, followed by precessing. Simultaneous diagnostics indicates that the entrainment of CVC leads to CH2O transport from the shear layer to the central region of the combustor. In general, the CH2O signal is mainly distributed in two positive velocity regions, the pilot/main jet and around CVC. Taking advantage of the CVC effect on radical transportation is a potential method to improve the mixing of the combustor, such as temperature distribution.

Experimental Investigation of Fluid-Structure Interaction in Mach 2 Flow Using Simultaneous High-Speed PIV and DIC

The fluid-structure interaction (FSI) of a compliant panel under a compression-ramp-induced shock-wave/boundary-layer interaction (SBLI) has been studied in Mach 2 flow. Simultaneous high-speed measurements of the velocity field and the panel displacement were conducted using 50 kHz particle image velocimetry (PIV) and 5 kHz stereoscopic digital image correlation (DIC). The mean effect of the panel displacement has been evaluated by monitoring the change in velocity profiles along the streamwise direction (x), upstream of the separated flow region. Streamwise (u) velocity near the panel surface has been shown to change its magnitude in response to the wall shape. Furthermore, the strong cross-correlation between fluctuations of the wall-normal panel displacement and the transverse (v) velocity can be explained by the flow remaining tangent to the wall surface as the panel deforms. This latter result is consistent with the panel motion being sufficiently low frequency compared to flow convective time scales that the flow is quasi-steady. In addition, assessment of the correlation between the separation shock position and panel displacement seems to suggest that when the panel is bulged down (concave up) at the downstream end of the panel, a larger separated flow is generated and the shock moves upstream. This observation remains speculative, but is consistent with the flow undergoing greater compression for the bulged down case.

Numerical and experimental study of flow structure and heat release distribution in a stratified swirl flame with high-speed PIV and OH* measurements

Stratified swirled flame is widely used in gas turbines and aero engines to achieve low emissions. However, the limitation of implementing laser diagnostic in real combustors acquires more accurate measurements of field information in the flame, especially the unsteady heat release, which relates to a lot of important phenomena, such as combustion instability and blow-off. The present study employs large eddy simulation (LES) combined with a detailed OH* chemiluminescence reaction mechanism to validate the chemiluminescent image of OH* in a stratified swirled flame at the atmosphere condition. 10 kHz particle image velocimetry images and OH* filtered images are recorded during the experiment. The heat release and flow structure of the stratified swirled flame at two different fuel stratification ratios show distinguished flame shapes. In general, the velocity results of LES have good agreement with the measurement. The numerical OH* and heat release comparison reveals a strong dependence on the local strain rate and turbulence level of OH* emissions. It is also noticed that the wrong flame shapes may be deduced from the Abel inversed OH* image since the signals are weak in the outer recirculation zone. This indicates that the strain rate in different regions of stratified swirl flame has a significant impact on OH* signal distribution. The results provide insight into the ability of chemiluminescent emissions, such as OH*, to indicate heat release in more complex industrial flames.

High-speed simultaneous PLIF/PIV imaging of a lift-off swirling flame under acoustic forcing

The detailed understanding of the flow-flame interaction in a swirling flame is crucial for the study of turbine engine combustion. The phase and amplitude of the heat release rate as a function of the frequency and amplitude of the flow-field oscillations determine the thermo-acoustic instability of the combustion system. The responses of a lift-off swirling flame under acoustic forcing, which operates near its blow-off limit and often shows flame shape bifurcation between attached and lift-off configurations, have not been experimentally investigated. Here, we present simultaneous 20 kHz particle image velocimetry (PIV), fuel tracer and CH2O planar laser-induced fluorescence (PLIF) measurement of a lean-premixed swirling flame enclosed in a model gas turbine burner in order to study the effect of equivalence ratio and forcing on the flame structure and dynamics. We studied the bistable states that have intermittent transitions between the anchored V-shape flame and the detached lift-off flame in which the precessing vortex core is formed. The time-resolved lift-off and re-attachment transitions between the V-flame and lifted flame configuration of the bistable flames are captured by the high repetition rate burst mode imaging. The high-repetition PLIF/PIV results demonstrate that the lifted flame can be re-attached on increasing the equivalence ratio or the forcing frequencies and amplitudes.

100 kHz PIV in a liquid-fueled gas turbine swirl combustor at 1 MPa

Simultaneous 100 kHz particle image velocimetry (PIV) and 10 kHz OH* chemiluminescence (CL) were used to investigate flow and flame dynamics in a liquid-fueled, swirl-stabilized, piloted burner operated at 1.0 MPa. The PIV measurements were focused on the inner recirculation zone (IRZ), which forms downstream of an annular bluff-body feature that separates the pilot and main reactant jets and plays a critical role in flame stabilization. The OH* CL images captured the large-scale flame structure and dynamics. For the operating condition studied in this work, a self-excited combustion instability was present and caused axisymmetric variations in flame length and global heat release at a frequency of 810 Hz. Dynamic Mode Decomposition (DMD) was performed on the measured velocity fields to isolate fluctuations associated with the thermoacoustic mode from other dynamic flow processes. The DMD results showed that the 810 Hz instability causes strong fluctuations of both the axial and radial velocity components within the shear layer between the pilot jet and IRZ, along with concomitant changes in the size and shape of the IRZ. The DMD spatial modes also revealed periodic generation of vortices within the pilot-IRZ shear layer at frequencies ranging from 4–5.5 kHz. Wavelet analysis was used to study the interaction between these smaller-scale flow structures and the large-scale flow-flame dynamics associated with the combustion instability. The frequency magnitude related to vortex shedding was shown to fluctuate with the 810 Hz instability. The peak frequency magnitude followed the maximum radial velocity in the shear layer, which occurs when the IRZ is at its farthest radial extent and the velocity gradients in the pilot-IRZ shear layer are the strongest. The characterization of cross-scale, flow-flame coupling in this highly turbulent, high-pressure flame was enabled by the state-of-the-art 100 kHz PIV measurements.

Ionic wind produced by a DC needle-to-plate corona discharge with a gap of 15 mm

The goal of the present study is to better understand the electrohydrodynamic (EHD) phenomena occurring in 15 mm gap point-to-plate corona discharges supplied by positive and negative DC voltages, and to link them to the discharge regime. For that, discharge current measurements have been conducted and a 20 kHz Particle Image Velocimetry system has been used to characterize the ionic wind produced by the discharges. The main results are as follows: (a) the Townsend’s law can always correctly interpolate the experimental I–V characteristics of a negative corona, even in presence of Trichel pulses, (b) in the case of a positive streamer discharge, the current does not follow the Townsend’s law as it evolves in Vk with k = 4 in the present study, (c) Indeed, the discharge current becomes higher than the Townsend’s law when the glow-to-streamer regime transition occurs, (d) the ionic wind is unsteady, more especially in the case of a positive corona for which it seems that its velocity is pulsed at the same frequency than the streamer one, (e) when the positive high voltage is switched on, a strong streamer occurs at the end of the voltage rising (t = 25 µs for +6 kV), resulting in an over-velocity region upstream the head of the ionic wind jet that progresses in quiescent air toward the plate, (f) the topology of the time-averaged ionic wind is fully different compared to the one observed in a previous study where the gap was equal to 25 mm, (g) in the case of a negative corona, the ionic wind velocity is nearly constant in the electrode gap, (h) in the case of the positive corona, the results are even more surprising since the velocity is minimum at the tip and increases when one approaches the grounded plate, indicating that there is a significant space charge remained by the streamers in the second half of the inter-electrode region, (i) generally speaking, this study highlights that the spatio-temporal characteristics of the EHD force and the resulting ionic wind depend on the distribution of the space charge between both electrodes, the latter being linked to the voltage polarity, the discharge regime and the electrode gap.

Dynamics of harmonically excited, reacting bluff body wakes near the global hydrodynamic stability boundary

This paper describes linear and nonlinear interactions between forced axial acoustic oscillations and the global mode of the reacting wake. This work is motivated by the problem of combustion instabilities, where acoustic oscillations associated with natural combustor modes excite hydrodynamic instabilities of the flow that, in turn, induce heat release oscillations. Wake flows with density stratification can be globally stable or unstable at high Reynolds numbers, and so the density change across the flame has significant influence on the natural flame and flow dynamics. Measurements were obtained in a facility in which flame density ratio, lip velocity and forcing frequency are independently varied using 5 kHz particle image velocimetry and Mie scattering measurements. By varying the density ratio, the hydrodynamic global mode growth rate can be systematically varied. In addition, measurements and analyses were performed where the forcing frequency is varied relative to the global mode frequency. While axial forcing excites a varicose response of the shear layers, the sinuous mode is the most rapidly growing. As expected, forcing at a frequency near the wake’s global mode frequency leads to rapid growth in vortical disturbance amplitude, and the symmetric vortices quickly stagger as they convect downstream leading to a large scale, sinuous flapping of the wake and flame. A linear, local stability analysis, together with a nonlinear analysis, help elucidate the physics that govern the vortex staggering. The study concludes with an analysis of the heat release dynamics. Significantly, the study shows that the heat release exhibits quite different sensitivities than the fluid dynamics; e.g. axial forcing of the flow near its global mode frequency leads to a reduction in heat release oscillations. This is true even though this forcing frequency maximizes the local degree of vortically induced flame flapping. Thus, the results of this study show some phenomena that contradict conventional notions, namely that conditions which align the frequency of a hydrodynamic global mode with that of an acoustic mode may lead to diminished forced heat release oscillations in bluff body combustors.

Simultaneous 5 kHz OH-PLIF/PIV for the study of turbulent combustion at engine conditions

Simultaneous measurements of velocity and scalar fields were performed in turbulent nonpremixed flames at gas turbine engine-operating conditions using 5 kHz particle image velocimetry (PIV) and OH planar laser-induced fluorescence (OH-PLIF). The experimental systems and the challenges associated with acquiring useful data at high pressures and high thermal powers are discussed. In this work, a wide range of operating conditions were studied, with a maximum pressure and thermal power of 1.8 MPa and 950 kW, respectively. In the PIV measurements, the high thermal power conditions were shown to cause significant defocusing of the particle images. This was the result of variations in the optical refractive index of the gas which were caused by strong temperature gradients within the inner structure of the flame. High flame luminosity also led to decreased SNR with increasing flame power. The OH-PLIF measurements did not show indication of strong laser sheet absorption at any condition tested. However, a decrease in the peak SNR was observed with increasing chamber pressure. An analysis of the true measurement resolution with respect to the scales of the flow is also given. Based on the resolved scales, the present dataset was used to study the time-averaged flow structure and its effect on flame behavior. Heat release conditioned flow statistics were studied to elucidate the flow–flame interactions in high-power flames.